It is known that metal particles of random shape and size can be formed by spraying the metal of interest in molten state into the upper part of a spray tower, by dropping molten metal onto a cooler surface, and by pouring a stream of molten metal into a bath of chilling liquid. These methods are satisfactory for the purposes for which they are intended, that is, for example, to produce metal particles suitable for grinding into powder for use in powder metallurgy, and to produce metal shot, such as lead shot. However, the products made by these known methods until the present invention have been of limited utility for use in soldering complex electronic components.
In soldering complex electronic components the solder particles used preferably have clean non-oxidized surfaces and are true spheres. For example, in soldering electronic devices such as chip carriers, preforms are put on the device and then reflowed. In order to assure the formation of reliable solder joints it is important that the preforms be spherical, uniform in size, clean and have a non-oxidized surface.
Until the present invention, such spheres were obtained by a very expensive process wherein solder discs were first stamped out of a solder plate and then reflowed in an inert atmosphere.